Generally, since a stimulated brillouin scattering-phase conjugate mirror reflects a phase conjugate wave, it can compensate for distortion of a laser beam occurring during a laser amplification procedure. Therefore, a stimulated brillouin scattering-phase conjugate mirror can be very easily applied to a beam combination high power laser. Examples of a beam splitting amplification system using stimulated brillouin scattering-phase conjugate mirrors are described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram showing a conventional wavefront-dividing amplification system using stimulated brillouin scattering-phase conjugate mirrors.
Referring to FIG. 1, light 505 emitted from a laser oscillator 500 is amplified and formed into more light beams while passing through a first light amplification stage 510, a second light amplification stage 540, and a third light amplification stage 570. That is, the light 505 is reflected from a Polarizing Beam Splitter (hereinafter referred to as a ‘PBS’), is incident on a first beam blocker 520, and is reflected from a Stimulated Brillouin Scattering-Phase Conjugate Mirror (SBS-PCM). The reflected light passes through the PBS again and is incident on a first light amplifier 530. Thereafter, light incident on the first light amplifier 530 is amplified while passing through the first light amplifier 530. The amplified light is reflected from the SBS-PCM and is incident on the PBS. The PBS reflects the incident light and outputs the reflected light to an optical expander 535. The optical expander 535 expands the incident light and outputs the expanded light to the second light amplification stage 540. The second light amplification stage 540 is provided with a second beam blocker 550, which has the same structure as the first beam blocker 520 of the first light amplification stage 510, and which performs the same function. However, the second light amplification stage 540 further includes a wavefront-dividing beam splitter 562 disposed upstream of a second light amplifier 560. The second light amplifier 560 is formed in a 2×2 array. In this case, the wavefront-dividing beam splitter 562 is used to individually transmit light beams to components constituting the 2×2 array of the second light amplifier 560. That is, before light is incident on the second light amplifier 560, the wavefront of the light is divided by the wavefront-dividing beam splitter 562, and divided light beams are amplified by respective amplifiers arranged along optical axes thereof. The amplified light beams are reflected from an SBS-PCM, and are combined and incident on the PBS. The PBS reflects the incident light and outputs the reflected light to a beam expander 565. The beam expander 565 expands the incident light and outputs the expanded light to the third light amplification stage 570. In the third light amplification stage 570, a third light amplifier 590 is formed in a 4×4 array, and a third beam blocker 580 is formed in a 2×2 array. In this case, wavefront-dividing beam splitters 582 and 592 are used to transmit light to respective arrays.
When such a light amplification system is constructed, a desired number of amplification stages are successively connected to each other, thus not only freely increasing output energy without causing damage to the optical system, but also maintaining a repetition rate at a uniform level. That is, when energy density is increased due to continuous amplification, the optical system and laser gain media may be damaged if energy density is not decreased. Therefore, the expansion of laser light is required, but the size of the laser gain media must also be increased. This results in a decrease in the cooling rate of the laser gain media, and thus it is actually impossible to generate laser light having a high repetition rate. Therefore, the beam combination amplification apparatus of FIG. 1, which employs a method of increasing the size of laser light, but maintaining the size of gain media unchanged, has been used.
FIG. 2 is a diagram showing a conventional amplitude-dividing amplification system using stimulated brillouin scattering-phase conjugate mirrors.
Referring to FIG. 2, the amplitude-dividing amplification system is constructed to include a laser oscillator 100 for generating laser light, a Beam Expander (BE) 101 for expanding the laser light, a PBS 102 for reflecting the expanded light, a first amplification stage 200 for amplifying the reflected light, a BE 103 for expanding the light amplified by the first amplification stage 200, a second amplification stage 300 for amplifying the light amplified by the first amplification stage 200 again, and a BE 104 for expanding the light amplified by the second amplification stage 300, and is constructed to allow the light amplified by the second amplification stage 300 to be output to a third amplification stage (not shown).
In the first amplification stage 200, devices for light amplification, that is, a quarter-wave plate 201, a PBS 202, a BE 203, a Faraday Rotator (FR) 204, an amplifier 205, an FR 206, and a Phase Locker (PL) 207, are arranged along an optical path. Further, in the second amplification stage 300, devices for light amplification, that is, a PBS 301, a quarter-wave plate 302, a PBS 303, a 45-degree rotator 304, a PBS 305, a BE 306, an FR 307, an amplifier 308, an FR 309, and a PL 310, are arranged along an optical path.
The light, which is output from the laser oscillator 100 and is S-polarized, is expanded by the BE 101 and is reflected from the PBS 102, and the reflected light is incident on the first amplification stage 200. The light incident on the first amplification stage 200 is converted into circularly polarized light while passing through the quarter-wave plate 201. The circularly polarized light is amplitude-divided by the PBS 202, and thus part of the circularly polarized light (P-polarized light) is reflected from the PBS 202 and the remaining part thereof (S-polarized light) passes through the PBS 202. P-polarized laser light and S-polarized laser light are individually amplified while passing through an optical path formed by the BE 203, the FR 204, the amplifier 205, the FR 206, and the PL 207, and then pass through or reflect from the PBS 202 in the same polarization states. The laser light beams are combined before the quarter-wave plate 201, and are circularly polarized, like that of the light before it was amplitude-divided. Thereafter, the circularly polarized light passes through the quarter-wave plate 201 and is converted into P-polarized laser light. Further, the laser light amplified in this way passes through the PBS 102 and is expanded through the BE 103.
Then, the light expanded by the BE 103 is incident on the second amplification stage 300. The second amplification stage 300 is operated to amplitude-divide the incident light into four light beams, combine the amplified light beams with each other, and output the combined light to a subsequent third amplification stage (not shown). The PBS 301 of the second amplification stage 300 outputs incident light to the quarter-wave plate 302. The PBS 303 amplitude-divides the light output from the quarter-wave plate 302, passes P-polarized laser light therethrough, reflects S-polarized laser light, and individually outputs the passed laser light and reflected laser light to the 45-degree rotator 304. In this case, in order to amplitude-divide a single laser light beam into two beams, a single 45-degree rotator 304 is connected to a combination of two PBSs 305, and thus a 2×2 array structure is formed. When the P-polarized and S-polarized light beams pass through the 45-degree rotator 304, the polarization of each beam is rotated by 45 degrees. Thereafter, each of the polarized beams is amplitude-divided into two beams by the subsequent optical device, that is, the PBS 305. A subsequent process is omitted because the same functions are performed on the same construction as that of the above-described first amplification stage 200. When the beams pass through and return to the 45-degree rotator 304, polarization is rotated by 45 degrees and then by −45 degrees, and thus there is no variation in polarization. Therefore, since amplitude division using the 45-degree rotator and the PBS can be infinitely performed, infinite energy amplification is possible if an additional amplification stage is provided after the second amplification stage 300.
The beam splitter used in the beam combination light amplification apparatus of FIG. 1 employs a wavefront dividing method, which is shown in FIG. 3a. 
As shown in FIG. 3a, a wavefront-dividing beam splitter splits incident light a into two output beams b. That is, the wavefront dividing method divides laser light into two small laser beams.
Meanwhile, the beam splitter may use the amplitude dividing method of FIG. 2 in addition to the wavefront dividing method. This method is shown in FIG. 3b. 
Referring to FIG. 3b, the amplitude-dividing beam splitter splits incident light a into two output beams b so that one output beam passes through the beam splitter and the other output beam is reflected from the beam splitter. That is, in the amplitude dividing method, two beams have only divided energy without changing the sizes thereof. Compared to the amplitude dividing method, the wavefront dividing method applied to the conventional light amplification apparatus cannot have a laser light shape identical to that of a main oscillator, so that there is difficulty in that the section of the gain medium of an amplifier must be processed in the shape of laser light. The reason for this is that, when the section of the gain medium is different from the shape of incident light, amplification efficiency may decrease. Further, the spatial distribution of laser light includes high spatial frequencies, and thus phase conjugation may be broken when reflection occurs through an SBS-PCM. Further, in the case where the phases of two beams are different from each other in areas where two beams intersect when the light beams are combined after being amplified, intensity spikes occur, thus deteriorating the spatial distribution of light.
However, since stimulated brillouin scattering is caused by random noise, a reflected beam has a random phase. Therefore, the combined laser beams have spatially different phase distributions. In the case of FIG. 1 (wavefront division), spike occurs at the boundary, and in the case of FIG. 2 (amplitude division), energy loss occurs. Therefore, in order to apply an SBS-PCM to a beam combination laser system, the phases of respective reflected beams are locked, and the phase difference between the phases must be zero.
Conventional methods of controlling the phases of the reflected beams of the SBS-PCM are described below.
FIG. 4 is a configuration diagram showing a conventional phase locking method based on a focus-overlapping method using an SBS-PCM.
Referring to FIG. 4, this method is implemented to focus a plurality of beams onto the scattering medium of a single SBS-PCM while overlapping the focuses of the beams with each other. That is, traveling beams are caused to pass through a condensing lens, and thus a plurality of beams is focused on the SBS-PCM while overlapping each other.
FIG. 5 is a configuration diagram showing a conventional phase locking method based on the back-seeding of a Stokes wave using a stimulated brillouin scattering-phase conjugate mirror.
Referring to FIG. 5, the term “Stokes wave” means a laser beam having the same frequency as the reflected wave, which is reflected by stimulated brillouin scattering. This method is implemented to allow a back-seeding laser beam to pass through a focus, thus amplifying the back-seeding laser beam. That is, an incident seeding beam is incident on the SBS-PCM after passing through an optical path formed by optical devices, and is reflected from the SBS-PCM, so that a back-seeding laser beam is generated and amplified.
FIG. 6 is a configuration diagram showing a conventional first method of locking the phases of laser beams using stimulated brillouin scattering and self-beam feedback, and FIG. 7 is a diagram showing a conventional second method of locking the phases of laser beams using stimulated brillouin scattering and self-beam feedback. That is, FIGS. 6 and 7 illustrate self-phase control methods of allowing incident laser beams to pass through SBS-PCM and to feed the laser beams back to stimulated brillouin scattering media using concave mirrors and Piezoelectric Transducers (PZT), thus controlling acoustic noise. In particular, the self-phase control methods are advantageous in that they can control the phases of stokes beams regardless of the number of beams.
However, in the beam combination laser system, short-term or long-term phase drift, occurring due to variation in density caused by the thermal effect of media on an optical path or the heterogeneity of density caused by convection current, and variation in optical path length caused by vibration and thermal expansion of optical devices, may be a problem, in addition to the random phases of stimulated brillouin scattering itself. However, in the conventional phase locking methods, since such a problem is not considered when the phase of a Stokes wave is controlled, the phase cannot be efficiently controlled.
Therefore, in order to sufficiently lock the phase of the reflected beam of the SBS-PCM, a new phase stabilization method is required in addition to the conventional phase control methods.